The conventional cryogenic refrigeration system is well-known by the Japanese Patent Laid-Open Gezette No. Sho 58-214,758, which expands high pressure gas, such as helium, in a refrigeration unit by moving a displacer having a hold over, thereby obtaining a very low temperature at a heat station adjacent to an expansion space of the gas.
Generally, this kind of cryogenic refrigeration system, as shown in FIG. 9, uses a low temperature expander EX having a plurality of heat stations H1, H2 and H3 to stepwise lower the cryogenic level, utilizes as a specimen mounting unit X2 the heat station H3 at the lowest temperature level provided at the end of the expander EX, and mounts a specimen, such as a super conductive element or a cryogenic thermometer, on the specimen mounting unit X2, thereby measuring various physical quantities.
The low temperature expander EX and specimen mounting unit X2 are contained in a vacuum container VC, thereby being heat-insulated from the atmosphere.
Furthermore, around the lowest temperature heat station H3 and specimen mounting unit X2 is disposed a thermal shield HS extending from the heat station H1 at the low-temperature expander EX, thereby shielding the specimen mounting unit X2 from heat radiation caused by the vacuum container VC in contact with the stmosphere and further demonstrating the adiabatic effect.
The aforesaid conventional refrigeration system, however, contains both the specimen mounting unit X2 and low temperature expander EX in one vacuum container VC, whereby, when a lid CA is open to exchange the specimen, a vacuum must be broken not only around the specimen mounting unit X2 but also around the low-temperature expander EX. Therefore, all the heat stations H1 through H3, as shown by the broken lines in FIG. 5, are required to raise the temperature up to substantially room temperature so as not to condence moisture in the atmosphere. Also, all the heat stations H1 through H3 after exchanging the specimen are required to be cooled down to a very low temperature, thereby taking much time for carrying out warm-up and cool-down.
In other words, when the specimen is exchanged, the entire vacuum container VC is broken of its vacuum and the first and second heat stations H1 and H2 as well as the specimen mounting unit X2 and third heat station H3, rise at the ambient temperature. A warming-up time until the temperature rise is finished, as shown by the broken line in FIG. 5, takes an extra time required to raise the ambient temperature of the first and second heat stations H1 and H2. When the lid CA is closed after exchanging the specimen and the vacuum container VC gets a vacuum to be cooled, the ambient temperature of the respective heat stations H1 through H3 is raised up to the room temperature, whereby it takes much time from a start to a finish of cooling down the heat stations.
Accordingly, all the heat stations as well as the specimen mounting unit X2 at the low temperature expander EX must be warmed or cooled, whereby energy is consumed in vain and the exchange of specimen takes much time, thereby creating the defect that the specimen cannot be frequently exchanged.